Why String s=null vs String s="null" with for loop gives the same value? [duplicate]

public static void main(String[] args) {
String s="";
String s1=null;
String s2="null";
for(int i=0;i<2;i++) {
s+=i;
s1+=i;
s2+=i;
}
System.out.println(s);
System.out.println(s1);
System.out.println(s2);
}

I understand why an empty string will give 01 as a result. But I don't understand why s1 and s2 values will be the same. While s1 is null and s2 has null as a string value. Still, they give the same answer.